Over the years, various cooling mechanisms have been employed to protect hot gas path components of gas turbine engines during extended operation, particularly stage one nozzles which often encounter the highest temperature exhaust gases. Most closed circuit cooling systems include a plurality of nozzle vane segments extending between inner and outer side walls of the nozzle. Typically, the vanes include cavities in fluid communication with compartments in the outer and inner side walls to accommodate the flow of cooling media within a closed circuit for cooling the outer and inner walls. The cooling media feeds into a plenum in the outer wall for distribution to the different chambers and flow passages defined by impingement openings allowing for the flow of coolant onto the outer wall surface of the vane. The spent impingement cooling media then flows into the leading edge and aft cavities extending radially through the vane.
In the past, steam has been used as a cooling medium for certain types of nozzle vanes. Even though steam has a higher heat capacity than air at nominal operating temperatures, steam cooling designs for turbine vanes and other engine components exhibit certain thermodynamic inefficiencies. For example, the steam must be maintained inside a closed circuit in order to avoid mixing with the hot gas stream. As a result, some components in the hot gas path cannot be cooled as efficiently with steam inside a closed circuit because, for example, the relatively thin structure of the trailing edges of the nozzle vanes precludes effective steam cooling of certain portions of the vanes.
Another known approach for cooling gas turbine engine blades and nozzles involves the use of a partial feed of high pressure cooling air, normally provided by an internal source such as an intermediate or final stage of a gas turbine compressor. Typically, a series of internal flow passages in and around the nozzle provide the desired supplemental cooling of the vanes using air film cooling and external piping supplies the compressed air to the nozzles which is eventually discharged into the hot gas stream of the gas turbine.
Most current gas turbines also rely on some form of impingement heat transfer to cool the nozzle vanes by placing a bank of round holes against the vane surface and introducing a relatively high velocity jet of fluid (steam or air) directly against the solid surface. The higher the velocity of the cooling fluid, the longer the molecules tend to remain in contact with the surface and exchange heat. For that reason, impingement cooling fluid jets normally introduce air perpendicular to the metal surface to maximize the incident velocity against the surface. In some recent designs, impingement air cooling has been used in combination with steam to lower the operating temperature of specific portions of the nozzle vanes that would not otherwise be effectively cooled by steam alone. However, virtually all impingement cooling systems for gas turbines using air alone rely on a prescribed number and arrangement of round holes in the vanes to accommodate the fluid flow.
Although relatively high levels of heat transfer can be achieved in a first stage nozzle using impingement cooling with round openings, once the impingement contact occurs, the fluid molecules tend to move parallel to the solid vane surface and the fluid velocity becomes significantly reduced with fewer molecules contacting the solid surface, ultimately resulting in reduced heat transfer. The cooling fluid velocity also becomes much lower due to fluid entering from neighboring round impingement holes which can collide, mix and eventually reduce the coolant throughput. Similarly, localized pressure sinks tend to redirect fluid flow, reducing the fluid velocity even further. This heat transfer degradation in nozzle vanes (called “cross flow effect”) decreases the level of heat transfer due to the phenomena invariably associated with round impingement openings.
Thus, it has been found that the use of compressed air and/or steam using round impingement holes for cooling purposes comes at a price of somewhat reduced thermodynamic efficiency due to the resulting air flow characteristics. The amount of heat transfer between coolant and the vane surface is directly proportional to the coolant velocity as it impinges and then turns parallel to the surface being cooled. Thus, a discrete set of varying heat transfer coefficients exist over the hot surface to be cooled. The highest heat transfer is achieved directly opposite the impinging hole but becomes lower as the coolant velocity decreases away from the hole. The cooling effect is also reduced by the cross flow interactions from adjacent round holes because the coolant from neighboring holes mixes with coolant from the round impingement hole, lowering its velocity and reducing the heat transfer potential.
A significant need therefore still exists to identify methods of maximizing the heat transfer potential of compressed air or steam used for vane cooling and thereby maintain component temperatures within strict operational requirements. A need also exists to increase the coolant velocity while making the flow more uniform over the largest area of the vane surface being cooled, thereby providing superior overall heat transfer efficiency.
As noted above, the current state of the art addressing impingement cooling issues relies almost exclusively on round impingement holes to produce a desired cooling effect. See, e.g., U.S. Pat. No. 6,468,031 (describing a nozzle using round impingement holes to increase the heat transfer on the internal face of the airfoil). Similarly, EP1247940A1 describes the use of round impingement holes having variable diameters to prevent clogging without reducing heat transfer between the coolant and nozzle surfaces.